1. Field of the Invention
The invention relates to motor protection circuits such as circuits for preventing restart of a motor for a predetermined duration after termination of the operation of the motor or circuits to stop operation of a motor when oil pressure fails for too long a duration. Such protection circuits can be employed in compressor motors for refrigeration or cooling systems.
2. Description of the Prior Art
The prior art, as exemplified in U.S. Pat. No. 3,435,289, No. 3,619,668, No. 3,660,718, No. 3,721,832, No. 3,721,866, No. 3,721,880, No. 3,742,303, No. 3,774,082, No. 3,794,858, No. 3,814,991 and No. 3,858,102, contains a number of motor protection circuits including circuits which employ timing facilities for preventing restarting of a compressor motor until after a predetermined delay from shut down. The prior art utilized timing circuits and devices directly connected with a control switch or the circuit energizing a contactor winding in order to initiate a delay period in response to a change in voltage conditions in the energization circuit for the contactor winding. One particular prior art motor protection system employed an electrical protection unit including a low oil pressure protection circuit, a thermal protection circuit, a delay after break circuit, a first opto-isolator relay responsive to the voltage across a contactor winding, and a second opto-isolator relay directly responsive to the voltage across one or more control switches in the contactor winding circuit, the first opto-isolator relay controlling the initiation of the time delay circuit upon cessation of voltage across the contactor winding, and the second opto-isolator relay preventing operation of the oil pressure protection circuit in the presence of a voltage condition across the control switches; however if the leads to the contactor winding and the control switches or the input power leads are reversed in the prior art motor protection system the opto-isolators would sense the wrong voltages and thus would render the system inoperable; the proper connection of such leads is subject to error as well as often being difficult during replacement of the electrical protection unit in existing systems because of obscured markings identifying the proper leads.
Current transformers having primary windings connected directly in the power lines to the compressor motor, or ferromagnetic current sensing loops coupled to the power line leads to the motor, have been utilized to sense the magnitude of current through the leads to operate a control circuit for the motor to prevent damage and to initiate a delay period after cessation of current to prevent restart until after an appropriate delay. However, such current transformers and current sensing devices must be installed in the heavy current leads to the motor which necessitates the installation of the current transformers or sensing devices external to electronic modular protection units thus increasing the number of components, complexity, size, and difficultness of repair of the system external to the modular electrical control unit.